Retroreflective materials are characterized by redirecting incident light back toward the originating light source. This property has led to the wide-spread use of retroreflective sheeting in a variety of conspicuity applications. Retroreflective sheeting is commonly applied to flat, rigid articles such as, for example, road signs and barricades to improve their conspicuity in poor lighting conditions. Retroreflective sheeting is also used on irregular or flexible surfaces. For example, retroreflective sheeting can be adhered to the side of a truck trailer, which requires the sheeting to cover corrugations and protruding rivets, or the sheeting can be adhered to a flexible body portion such as a road worker's safety vest or other such safety garment.
Many conspicuity applications are governed by specific performance standards for retroreflective sheeting. Manufacturers must demonstrate that their retroreflective sheeting is capable of meeting the relevant performance standards to be considered as a supplier in the marketplace. A body of standards exists both for describing retroreflection (see ASTM Designation E808-93b, Standard Practice for Describing Retroreflection) and for measuring retroreflectors, (see ASTM Designation E809-94a, Standard Practice for Measuring Photometric Characteristics of Retroreflectors).
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as "beaded" sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. Nos. 3,190,178 (McKenzie), 4,025,159 (McGrath), and 5,066,098 (Kult). Due to the symmetry of beaded retroreflectors, microsphere-based sheeting exhibits relatively uniform entrance angularity when rotated about an axis normal to the surface of the sheeting. Therefore, the retroreflective performance of beaded sheeting is relatively insensitive to the orientation at which the sheeting is placed on the surface of an object. In general, however, microsphere-based sheeting exhibits relatively low retroreflective efficiency. Beaded retroreflective sheeting typically exhibits a total light return of approximately 5% to 15% in an observation cone angled about 2.degree..
Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar base surface and a structured surface comprising a plurality of cube corner elements opposite the base surface. Each cube corner element comprises three mutually substantially perpendicular optical faces that typically intersect at a single reference point, or apex. The base of the cube corner element acts as an aperture through which light is transmitted into the cube corner element. In use, light incident on the base surface of the sheeting is refracted at the base surface of the sheeting, transmitted through the respective bases of the cube corner elements disposed on the sheeting, reflected from each of the three perpendicular cube corner optical faces, and redirected toward the light source.
One aspect of many performance standards requires retoreflective sheeting to return specified percentages of light incident on the face of the sheeting at various entrance angles. The total light return characteristic of a retroreflective sheeting as a function of the entrance angle of incident light is generally referred to in the art as the `entrance angularity` of the sheeting. A retroreflective sheeting capable of returning a significant percentage of light incident at relatively high entrance angles can be characterized as having strong or wide entrance angularity, such as disclosed in the isobrightness curves in U.S. Pat. No. 4,588,258 (Hoopman).
By contrast, retroreflective sheeting with poor entrance angularity loses its retroreflective brightness (total light return decreases) rapidly as the angle of incidence deviates from 0.degree.. Moreover, entrance angularity typically varies about a 360.degree. range of orientation angles (orientational uniformity), requiring proper alignment of the retroreflective sheeting for each application. The entrance angularity and orientational uniformity of a retroreflective sheeting is a significant performance factor because it materially affects the ability of a driver to see an object such as a traffic sign or a safety barrier in poor lighting conditions at various orientations.
The symmetry axis, also called the optical axis, of a cube corner element is the axis that forms an equal angle with the three optical surfaces of the cube corner element. Cube corner elements typically exhibit the highest optical efficiency in response to light incident on the base of the element roughly along the optical axis. The amount of light retroreflected by a cube corner retroreflector drops as the incidence angle deviates from the optical axis.
Cube corner elements offer the advantage of being significantly more efficient retroreflectors than beads. The terms `active area` and `effective aperture` are used in the cube corner arts to characterize the portion of a cube corner element that retroreflects light incident on the base of the element. A detailed teaching regarding the determination of the active aperture for a cube corner element design is beyond the scope of the present disclosure. One procedure for determining the effective aperture of a cube corner geometry is presented in Eckhardt, Applied Optics, v. 10, n. 7, July, 1971, pp. 1559-1566. U.S. Pat. No. 835,648 (Straubel) also discusses the concept of effective aperture. At a given incidence angle, the active area can be determined by the topological intersection of the projection of the three cube corner faces onto a plane normal to the refracted incident light with the projection of the image surfaces for the third reflections onto the same plane. The term `percent active area` is then defined as the active area divided by the total area of the projection of the cube corner faces. The retroreflective efficiency of retroreflective sheeting is directly proportional to this percent active area. The maximum theoretical total light return of truncated cube corner elements commonly used in retroreflective sheeting is approximately 67%, while in practice cube corner retroreflective sheeting exhibits a maximum total light return of approximately 35%, due to sealing, front surface losses, and reflection losses at the cube faces.
Predicted total light return (TLR) for a cube corner matched pair array can be calculated from a knowledge of percent active area and ray intensity. Ray intensity can be reduced by front surface losses and by reflection from each of the three cube corner surfaces for a retroreflected ray. Total light return is defined as the product of percent active area and ray intensity, or a percentage of the total incident light which is retroreflected. A discussion of total light return for directly machined cube corner arrays is presented in U.S. Pat. No. 3,712,706 (Stamm).
The light return profile of the basic cube corner element is inherently asymmetric in nature. The breakdown of total internal reflection (TIR) is the most significant cause of this asymmetry in non-metallized cube corner retroreflectors. Coating the reflecting faces with a specular reflector substantially reduces the asymmetry in the reflection pattern. Metallized cube corner arrays, however, are typically not white enough for daytime viewing, such as on signing applications. The durability of the specular vapor coat may also be inadequate. Finally, a portion of the asymmetry is due in part to the asymmetric physical geometry of a cube corner element. See Rityan, Optics of Corner Cube Reflectors, Soviet Journal of Optics Technology, v. 34, p. 195 (1967).
Retroreflective sheeting formed from cube corner elements exhibits a corresponding asymmetry in its light return profile. By way of example, U.S. Pat. No. 3,712,706 to Stamm ('706 patent) discloses the three-lobed light return profile characteristic of a single cube corner element. Similarly, U.S. Pats. No. 4,202,600 (Burke) and 4,243,618 (Van Arnam) disclose an array of cube corner elements having a plurality of zones with different angular orientations, such that the total light return retroreflected by cube corner retroreflective sheeting varies as a function of the entrance angle of the incident light and the orientation angle of the sheeting on the substrate. The six-lobed light return profile of Burke and Van Arnam is characteristic of a matched pair of cube corner elements.
One approach to reducing the asymmetry of retroreflective sheeting is by providing a retroreflective sheeting construction with a plurality of discrete cube corner arrays disposed at different orientations; a technique referred to in the art as `tiling`. Burke and Van Arnam patents disclose retroreflective sheeting having arrays of conventional truncated cube corner elements with equilateral base triangles tiled in a variety of different orientations on the surface of the sheeting. While the constructions suggested in these references address the issue of asymmetry, the cube corner geometries disclosed in these references suffer a rapid decline in total light return at entrance angles greater than about 40.degree., since only a small portion of the cubes are optically functional at a particular orientation. Therefore, retroreflective sheeting in accordance with these references do not provide adequate total light return at high entrance angles for many applications.
Another approach to accommodating this variation in entrance angularity is to design retroreflective sheeting to have specific planes of improved entrance angularity. By way of example, the Hoopman patent discloses a retroreflective sheeting wherein the cube corner elements are arranged in opposing matched pairs having their respective symmetry axes tilted toward one another. This geometry results in a retroreflective sheeting with improved entrance angularity in a plane substantially coincident with the plane that contains the symmetry axes of the cube corner elements, identified as the X-axis plane, and also in a Y-axis plane perpendicular to the X-axis plane. In use, the sheeting is preferentially oriented on the substrate such that these planes coincide with the planes in which light will become incident on the sheeting. By way of example, a preferred orientation for the sheeting on a road sign is to align the X-axis plane substantially parallel with the ground.
U.S. Pat. No. 5,565,151 (Nilsen) discloses matched pairs of retroreflective cube corner elements that are tilted or canted between more than 1.0 degree and less than about 7.0 degrees in a negative direction. A section of one of the cube corner elements in the matched pair is removed, creating a smaller element which produces increased observation angle performance.
U.S. patent application Ser. No. 08/5887,719 (Nestegard et al.), entitled Dual Orientation Retroreflective Sheeting, discloses a retroreflective sheeting with alternating zones of cube corner arrays oriented such that their primary planes of entrance angularity are approximately perpendicular to one another.
Thus, there is a need in the art for a retroreflective sheeting that maintains a visibly useful total light return across a 360.degree. range of orientation angles, particularly at entrance angles greater than about 40.degree.. Additionally, there is a need in the art for a retroreflective sheeting having relatively small variations in total light return across a 360.degree. range of orientation angles at higher entrance angles and particularly at entrance angles greater than about 40.degree..